A liquid crystal display device is a system of displaying images by using the electro-optic effect based on the orientation of the liquid crystal molecules, and controlling the amount of transmission of the light that is irradiated from the back surface, and generally speaking a planar illuminating device referred to as a backlight unit configured from a cold-cathode tube or the like is required. In recent years, this kind of liquid crystal display device is being manufactured as larger screens, and liquid crystal display devices for TV use of 50-inch sizes and larger have been put into practical use. Nevertheless, since the power consumption is increasing pursuant to the enlargement of the liquid crystal display device, the development of technology capable of realizing low power consumption is being demanded. Moreover, in order to reduce the space that will be occupied by the liquid crystal display device in a room, thinner liquid crystal display devices are also being strongly demanded.
In order to meet the demand for such thin liquid crystal display devices of low power consumption, the use of light-emitting diodes (hereinafter referred to as the “LED”) with favorable luminous efficiency and laser as the light source, and an edge light configuration using a thin light guide plate are being considered. Among the above, a liquid crystal display device of an edge light configuration using the LED as its light source has already been put into practical application.
Moreover, in order to realize low power consumption, consideration is also being given to improving the transmittance of a liquid crystal panel which was conventionally 5% or so. As a major factor that deteriorates the transmittance in a liquid crystal panel, there is a polarizing plat disposed on the backlight unit side of the liquid crystal panel, and a color filter that selectively transmits red light, green light, and blue light for each sub-pixel. Normally, when non-polarizing white line enters the liquid crystal panel, the amount of transmitted light will be ½ with the polarizing plate disposed on the entrance plane. In addition, since red light, green light, and blue light are selectively transmitted for each sub-pixel with the color filter, the amount of transmitted light that became ½ will be further reduced to ⅓ or less thereof.
Among the above, as a method of improving the transmittance with the polarizing plate disposed on the backlight unit side, a configuration of inserting a polarized reflection sheet between the liquid crystal panel and the backlight unit, causing necessary polarization components to be transmitted, reflecting the unwanted polarization components, rotating the polarization of the reflected light, and converting this to a valid polarization component for reuse has been put into practical application.
Moreover, as a method of improving the transmittance with the color filter, proposed is a configuration of using a wavelength separation means such as a diffraction grating to separate the irradiated light that was output from the backlight unit into red light, green light, and blue light, and efficiently focusing them to predetermined sub-pixels of the liquid crystal panel (for example, refer to Patent Documents 1, 2, and 3). In addition, proposed is a configuration of performing wavelength separation of red light, green light, and blue light with an interference filter-type mirror with wavelength selectivity formed on the inside of the light guide plate, and efficiently guiding the light to the predetermined sub-pixels of the liquid crystal panel (for example, refer to Patent Document 4).
Moreover, improvement of the color filter itself is also being considered, and a color filter having a structure that is different from a conventional absorptive color filter has been proposed. For example, proposed is a color filter configured so that the film thickness of a part of the film structure of the interference filter is changed for each region to realize different spectroscopic properties for each region (for example, refer to Patent Document 5). Patent Document 5 aims to improve the color reproducibility by using this color filter.
Furthermore, proposed is a configuration of adding wavelength characteristics to the wire grid polarizing plate or combining the polarization characteristics and wavelength characteristics by using fine grating of subwavelength, configuring a polarization color filter in which the function of the reflective polarization filter and the function of the color filter are integrated, and using this to guide polarized light of a predetermined wavelength to predetermined sub-pixels of the liquid crystal panel (for example, refer to Patent Documents 6 and 7). According to this configuration, the efficiency of the liquid crystal panel can be improved considerably.
Nevertheless, with the foregoing conventional configurations, no specific configuration has been presented for reducing power consumption, or the effect of reducing power consumption is insufficient.
Specifically, Patent Documents 1 to 3 describe a configuration of performing wavelength separation using diffraction and guiding light to the respective sub-pixels of the liquid crystal panel, but Patent Documents 1 to 3 are all unable to sufficiently improve the light utilization efficiency, and further improvement in efficiency is difficult. Specifically, with Patent Documents 1 and 2, since red light, green light, and blue light are separated with a diffraction angle difference of approximately several degrees, it is necessary to shorten the grating pitch to approximately several times the wavelength. With this kind of configuration, it is difficult to sufficiently improve the diffraction efficiency. In addition, with Patent Document 3, since a volume hologram is used, it is possible to perform wavelength separation at a predetermined angle difference. However, since the incidence angle to the volume hologram will be limited, the output efficiency of light that entered the light guide plate will deteriorate.
Moreover, although Patent Document 4 shows a configuration of forming a mirror for performing wavelength separation inside the light guide plate, it is difficult to manufacture a light guide plate having this kind of structure, and there is concern that costs will increase.
Moreover, the color filter described in Patent Document 5 is merely used in substitute for a conventional absorptive color filter, and Patent Document 5 fails to show a configuration of effectively using the reflected light.
Patent Documents 6 and 7 describe using a polarization color filter and reusing light that was reflected off the polarization color filter. Nevertheless, the effect of this polarization color filter is insufficient when used as a substitute for the polarizing plate and color filter of the liquid crystal panel, and, even though the light utilization efficiency can be improved to a certain degree, the picture quality will deteriorate.
Specifically, with the polarization color filter of Patent Document 6, in comparison to a conventional color filter, the transmittance of green light and red light in the blue filter region and the transmittance of red light in the green filter region are high, and the color reproducibility will deteriorate considerably. Moreover, with the polarization color filter of Patent Document 7, transmittance of the unwanted polarization component is 60% or more of the required polarization component, and the contrast will deteriorate since the extinction ratio of light that is guided to the pixels of the liquid crystal panel is inferior.    Patent Document 1: Japanese unexamined Patent Application No. 2000-241812    Patent Document 2: Japanese unexamined Patent Application No. H9-113903    Patent Document 3: Japanese unexamined Patent Application No. H10-253955    Patent Document 4: Japanese unexamined Patent Application No. 2006-12722    Patent Document 5: Japanese unexamined Patent Application No. 2008-170979    Patent Document 6: Japanese unexamined Patent Application No. 2007-25692    Patent Document 7: WO 2007/034827